1. Field of the Invention
This invention relates to vehicle cab suspension systems of the type which employ springs and shock absorbers to support a vehicle cab, such as a truck cab, on the frame of the vehicle, and in particular to a crash restraint for such a system.
2. Discussion of the Art
Truck cab suspensions which employ springs, typically air springs, and shock absorbers are well known for supporting the rear of truck cabs of the type that are hinged at the front of the cab to the truck frame along a lateral axis. For example, U.S. Pat. Nos. 4,989,684; 5,109,939; 5,209,316; and 5,253,853 disclose such suspension units. These units usually have two spaced plates, with an upper one of the plates fixed to the cab and a lower one of the plates fixed to the frame, with the plates vertically movable relative to one another. The upper plate is supported by the springs and damped by the shock absorbers. In addition to these components, the suspension units usually have an air valve for regulating the height of the unit by regulating the admission or exhaust of air to the air springs, and may have a track arm, as shown in the '853 patent, for controlling the lateral, or side-to side, tracking of the cab relative to the frame, to restrain the cab laterally relative to the frame.
When a truck with a suspension unit as described above has a crash, the frontal impact causes the cab to rotate forwardly about the frontal pivot axis. This causes a much greater separating force on the plates of the suspension unit than is normally encountered in over-the-road usage. The force is so great that the suspension unit can be simply pulled apart, which can result in the cab pivoting frontwards beyond the normal intended limit. Consequently, a need exists for a suspension unit with better restraint of the cab under crash conditions.